Biodiesel from biologically derived oils and fats have been attracting increased attention due to its potential of providing a significant portion of transportation fuels. Biologically derived oils and fats are complex mixtures of triglycerides and free fatty acids. Transesterification, i.e. reacting triglycerides with methanol to produce fatty acid methyl-ester (FAME), is used to make biodiesel from biologically derived oils and fats. See, for example, Huber et al., “Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering,” Chem. Rev., vol. 106, pp. 4044-4098, 2006; and Antolin et al., “Optimisation of Biodiesel Production by Sunflower Oil Transesterification,” Bioresource Technology, vol. 83 (2002), pp. 111-114. FAMEs generally have a high cetane number and are considered to burn cleanly, but they still contain problematic levels of oxygen. A major drawback of this type of biodiesel is that it generally has poor oxidative and thermal stability.
In order to improve the energy density and stability of this type of biodiesel, oxygen must be at least partially removed. Hydrotreating is one route to remove oxygen from triglycerides, but it has the disadvantage that it consumes large amounts of hydrogen since during hydrotreating, oxygen is reacted with hydrogen and removed through the formation of water. The heat release from hydrotreating reactions is also a significant challenge for reactor design. Decarboxylation is another route to remove oxygen from biofuels. Although the hydrocarbons produced through decarboxylation may contain less carbon than its fatty acid or ester counterpart and those from the hydrotreating route, decarboxylation does not consume hydrogen. Generally speaking, consuming the carbon contained in a biologically derived feedstock to remove oxygen is less costly than consuming hydrogen which must be produced through expensive processes.
There have been reports in the literature of biofuel production through decarboxylation and decarbonylation of biologically derived oils using supported noble metal catalysts at relatively high temperatures, e.g., for example, greater than 350° C. For example, platinum (Pt) supported on carbon had been found to be an effective catalyst. However, carbon monoxide (CO) produced from the reaction can poison the Pt catalyst. Hence, high hydrogen partial pressure is needed to keep the catalyst surface clean and reduce catalyst deactivation.
It would be desirable to have a more economical process for producing transportation fuels from biologically derived oils while avoiding the aforementioned difficulties.